Basically the easiest trick is to grind as you go. There may be times when you've ended up using a party member more than others and it's significantly stronger. Bring the rest of your party up to its level before you continue or you'll end up with one ridiculously strong Pokemon and a so-so party.

Grinding's pretty simple. Use move type advantages when you can and aim for things that are common but yield decent experience. It's better to take down 50 Pidoves instead of solely hunting for an Audino. Also if you're playing a game with a VS Seeker, don't be afraid to abuse it.